Towers forced-draft

Cooling tower (Forced draft) Water flowrate (m3-h- ) 10 4.43 x 103 10-40 0.63... 

Plume. Condensed visible water vapor in air discharged from tower. Forced draft ... 

Two types of mechanical-draft towers are in use today the forced-draft and the induced-draft. In the forced-draft tower the fan is mounted at the base, and air is forced in at the bottom and discharged at low velocity through the top. This arrangement has the advantage of locating the ran and drive outside the tower, where it is convenient for inspection, maintenance, and repairs. Since the equipment is out of the hot, humid top area of the tower, the fan is not subjected to corrosive conditions. However, because of the low exit-air velocity, the forced-draft tower is subjected to excessive recirculation of the humid... 

Forced Draft Air introduced at the bottom of the tower is forced to the top by a centrifugal blower. 

Natural and forced-draft cooling towers are generally used to provide the cooling water required on a site, unless water can be drawn from a convenient river or lake in sufficient quantity. Seawater or brackish water can be used at coastal sites, but if used directly will necessitate more expensive materials of construction for heat exchangers. 

Figure 9-102. Cross-section of iow-head forced-draft tower showing fan housing arrangement, fiiling, water distribution spray system and spray eliminators. Used by permission of Foster Wheeler Corp., Cooling Tower Dept.

Recirculation the portion of exit or outlet air from the tower that recirculates back to the inlet of the fresh air to the tower. To keep this low it is important to space towers away from each other as well as from any structures which can deflect the exit moist air back to the inlet. Due to recirculation the wet bulb temperature at the tower inlet may be different from that at a point 100 yards away. The recirculation of induced draft towers is usually less than forced draft due to the upward velocity of discharge of the air. 

Normal recirculation in average installations for forced draft may run 3-10% of total inlet air, and 1-8% for induced draft towers, all depending upon the location and wind conditions during any day or season. Some towers can be arranged to have less than 1% recirculation. If conditions are suspected of being conductive to recirculation, it should definitely be allowed for in design of the tower. Recirculation increases the wet bulb temperature of entering air, increases the total air required (and hence size of... 

The tower pressure losses are (1) tower packing or fill (70-80% of loss) (2) air inlet if induced draft (3) mist eliminators at top (4) air direction change losses and entrance to packing on forced draft units. These losses are a function of air velocity, number and spacing of packing decks, liquid rate and the relation between L and Ga. 

Water quality is important, not only from an environmental point of view but also in relation to the type of packing to be specified. Analysis of the circulating water is simple to obtain, but it is very seldom offered to the cooling tower designer. The quality, or lack of it, will determine the type of pack to be used, the selection of structural materials and whether the tower should be induced or forced draft, counterflow or crossflow. Water treatment, in the shape of chemicals to control pH and to act as counter-corrosion agents or as biocides, all has a bearing on tower selection. 

Remember that outside influence (for example, new building work in the vicinity of the installation) can increase air-based pollution, such as cement dust entering the tower at air inlet levels or via the forced-draft fan. 

Mix products - placing a forced-draft tower beside an induced-draft one causes problems for both designs (Figure 34.12). 

Figure34.12 Mixing forced draft with induced. The overloaded forced-draft tower with excess plume results in elevated wet bulb at air inlets on new tower. Removing the forced draft and adding one more cell to the induced draft resolved the problem...

Delta Cooling Towers, Inc. (Delta), has designed and manufactured two complete air stripper systems VANGUARD and Aqua-Trim . The air strippers use a countercurrent, forced-draft design to remove volatile organic chemicals and certain other substances from water. While the VANGUARD air stripper is commercially available. Delta has stopped making the Aqua-Trim stripper. 

Figure 8.4. Example of tubular heat exchangers (see also Fig. 8.14). (a) Double-pipe exchanger, (b) Scraped inner surface of a double-pipe exchanger, (c) Shell-and-tube exchanger with fixed tube sheets, (d) Kettle-type reboiler, (e) Horizontal shell side thermosiphon reboiler, (f) Vertical tube side thermosiphon reboiler, (g) Internal reboiler in a tower, (h) Air cooler with induced draft fan above the tube bank, (i) Air cooler with forced draft fan below the tube bank.

Figure 9.18. Main types of cooling towers, (a) Atmospheric, dependent on wind velocity, (b) Hyperbolic stack natural draft, (c) Hyperbolic assisted with forced draft fans, (d) Counterflow-induced draft, (e) Crossflow-induced draft, (f) Forced draft, (g) Induced draft with surface precooler for very hot water also called wet/dry tower, [(fc)-(e) from Cheremisinoff and Cheremisinoff, 1981).

Air paths may be crossflow or counterflow. Fan placement can be ahead of the fill section (forced draft) or behind it (induced draft). Manufacturers use different criteria in sizing units. Figure 4.18 reviews one tower manufacturer s approach. Specific design details should be obtained from the tower manufacturer. 

Figure 4.18 Upper right figure shows a forced-draft or blowthrough tower, which has a fan at the bottom for driving air through the fill above, Tower selection for smaller units can be made from the accompanying curves and table for a cold water temperature of 85°F (this is generally the water basin discharge temperature for small towers). As an example, enter at 104°F hot water temperature to a wet bulb value of 75°F, then drop vertically to the water flow selected (580 gpm). This falls between curves that designate the manufacturer s distinct model size. Select the next larger size, i,e., the curve immediately below, and follow across to the recommended tower model).

The following data have been obtained for a forced-draft cooling tower ... 

Under some wind conditions, a portion of the warm moist air leaving the tower may recirculate back through tire tower inlet and thus degrade performance. Forced-draft towers have recirculation rates that are about double those of induced-draft towers. Both water loading and tower height play the dominant role in- recirculation. Correlations exist in the literature for defining the effects of these parameters, and corrections can be applied to the wet-bulb temperature [2,3], Cooling tower fabricators can supply data to estimate the severity of the problem. 

Induced- or forced-draft towers usually have a shell of wood or cement asbestos, a framework of wood and wooden slat fill. Some towers are entirely noncombustible, with a shell of galvanized sheet steel or cement asbestos, a framework of steel and with either spray nozzles or fill materials proven by appropriate fire tests to be nonhazardous. 

The paper considers the state-of-the-art in cooling towers, covering various types of towers in use. It discusses how they respond to the present and the future needs of the industry. A trend toward the counterflow design in the heat exchanger is indicated, and a forced draft counterflow tower is described. The design of the fan-assisted tower using both mechanical and natural draft is briefly dealt with. 1 ref. cited. 

On the Minimum Size For Forced Draft Dry Cooling Towers for Power Generating Plants... 

Fig. 1.3 (a) Natural draft, hyperbolic, counterflow tower (b) Natural draft, hyperbolic crossflow tower (c) Induced draft, crossflow tower (d) Induced draft, counterflow tower (e) Forced draft, galvanized, evaporative condenser (f) Induced draft, FRP, bottle tower. 

Galvanized Steel Blow-Through (Forced Draft) Cooling Towers... 

Is there any ice damage (particularly in forced draft towers) ... 

Enclosed air space between drift eliminators in induced towers and between fan and fill in forced draft towers. 

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